Conventional technology will be described with respect to electron-beam reduction-transfer technology.
Electron-beam reduction-transfer apparatus are representative microlithography devices that project a pattern, such as of an integrated circuit, onto a suitable substrate such as a semiconductor wafer. The pattern is defined by a mask that is irradiated by an electron beam. Electrons passing through the mask are capable of forming an image of the irradiated portion of the pattern. The image is projected using a two-stage projection lens (projection-optical system) onto the substrate. In this regard, reference is made to, e.g., Japan Kokai patent document no. Hei 5-160012. In such an apparatus, the electron beam normally cannot collectively irradiate the entire mask region at one time. Consequently, the field of view of the projection-optical system is typically smaller than the mask pattern, and the mask pattern is typically divided (segmented) into multiple small regions (mask subfields) that are individually projected onto the substrate. In this regard, reference is made to U.S. Pat. Nos. 5,260,151 and 5,466,904.
Several configurations of the projection-optical system are known, e.g., MOL (Moving Objective Lens) and VAL (Variable Axis Lens) configurations. Both configurations involve movement (e.g., lateral shifting) of the optical axis of the projection lens by application of a lateral magnetic field (using an appropriate deflector) to the magnetic field generated by the projection lens. The projection lens is typically in a symmetrical magnetic doublet (SMD) configuration that is effective in substantially reducing certain aberrations.
An SMD lens system is a "two-stage" design comprising two separate lenses each having a respective pole piece bore diameter and lens gap. The lenses are coaxial and arranged in tandem on the optical axis. In terms of overall configuration, each lens is the mirror image of the other about a location on the optical axis between the lenses. Normally, the downstream, or second, lens (situated closer to the substrate) is dimensionally reduced in size, compared to the upstream, or first, lens (situated closer to the mask), by the demagnification ratio of the lens system. Also, the magnetic field generated by the first lens has an opposite polarity to the magnetic field generated by the second lens. Each lens has a respective main electrical coil. The ampere-turn values of each main coil are normally equal. J. Vac. Sci. Techol. 12(6) November/December, 1975). The SMD configuration is effective in canceling .theta.-direction aberrations and distortions and reducing chromatic aberrations of magnification and rotation essentially to nil.
In order to ensure adequate positional accuracy of the transferred image using a conventional apparatus as described above, conventional wisdom dictates that a separate power supply having an output precision of at least 10.sup.-6 be used to energize each coil of each lens and deflector of the electron-beam optical system (including irradiation-optical system and projection-optical system). Thus, individual high-precision power supplies are conventionally connected to each lens of the two-stage lens system in order to achieve adequate control of image rotation and magnification.
In the face of contemporary demands for ever higher throughput and precision, the field-of-view dimensions of the electron-beam optical system on the mask is about 10 mm, and positional stability of the electron beam is 5 nm or less. To achieve such specifications, the stability of each power supply must be 5.times.10.sup.-7 or less.
Changing the beam-deflection angle as effected by a deflector involves a change in the electrical current applied to the respective deflector. Such an operation is required each time, for example, a different mask subfield is being illuminated. Each such change requires sufficient settling time of the power supply used to energize the deflector. During such a settling period, no actual projection transfer can be performed.
When performing pattern transfer of a segmented mask as summarized above, the mask subfields are selected and exposed at a high speed (preferably 25 .mu.sec or less per subfield). To achieve such performance, many separate and highly stable power supplies are utilized that can be controlled at high speeds (one power supply per deflector). This results in high cost.